1. Field of the Invention
The present invention relates to servo control of an optical disc apparatus that records and plays back information on and from an optical disc by means of an optical pickup, and particularly, to focusing servo control.
2. Description of the Background Art
Conventionally, there has been an optical disc apparatus that records and/or plays back information, by means of an optical head referred to an optical pickup, on and/or from an optical disc having concentric or spiral information recording track(s) formed thereon, such as a CD or a DVD. The optical pickup is configured to condense and apply light of a semiconductor laser or the like on the optical disc by an objective lens, and to receive reflected light of the applied light from the optical disc to output a corresponding electric signal, and it moves in the radial direction of the optical disc.
The optical disc apparatus moves the optical pickup to a prescribed position on the optical disc. While rotating the optical disc, the optical disc apparatus condenses the light from the optical pickup and applies it to the recording track of the optical disc, thereby records and/or plays back information on and/or from the optical disc.
When information is played back from an optical disc, firstly, focus-on (focus lock-in) is attained by focusing servo control so that the focusing point of the light applied by the optical pickup is positioned on the surface of the optical disc. Then, track-on (track lock-in) is attained by tracking servo control so that the focused-on light is positioned on the recording track. Such focus-on and track-on are performed by moving the objective lens in the direction perpendicular to the optical disc surface and in the direction perpendicular to the recording track. In the state where focus-on and track-on are attained, based on an electric signal output from the optical pickup, pits formed on the optical disc are detected, whereby information is read from the optical disc, and the information is played back. The recording of information on the optical disc is performed by forming pits in the recording track of the optical disc by the light condensed and applied to the optical disc from the optical pickup.
In playing back and recording information from and on an optical disc, the above-described focusing servo control and tracking servo control are performed by obtaining a focus error signal and a track error signal by calculation based on reflected light of the laser light, and by adjusting so that the errors are reduced. The adjusting operation greatly changes the record signal quality (such as jitter), and therefore precise adjustment is required. Japanese Patent Laying-Open Nos. 2000-222747, 2002-269773, and 2005-216461 disclose related techniques.
On the other hand, a DVD+R or a DVD+RW as one type of optical discs employs the wobble groove scheme as the physical format. Specifically, a spiral groove is provided, along which data is written. The groove is wobbled such that it slightly meanders cyclically in the radial direction. In the wobble, address information that is ADdress In Pre-groove information indicative of the physical address is embedded.
When information has been recorded on the optical disc and thereafter the ADdress In Pre-groove information embedded in the wobble is read, conventionally the ADdress In Pre-groove information is read using a focus balance value set by a focus control unit as it is, as in the case of reading normal recording pits.
However, after the information has been recorded on the optical disc, reading the address information embedded in the wobble may be difficult due to the effect of superimposition components of the recorded RF (Radio Frequency) signal.
FIG. 5 is a graph for describing the relationship between the jitter value of an RF signal (also referred to as an RF jitter value) generated by reading pits recorded on an optical disc or an ADdress In Pre-groove error rate and a focus balance value.
Referring to FIG. 5, the left ordinate axis indicates RF jitter value. In connection with the focus balance value, focus balance value Fb with which the RF jitter value is minimized is shown. The right ordinate axis indicates ADdress In Pre-groove error rate, that is the error rate in reading ADdress In Pre-groove information. In connection with the focus balance value, focus balance value Fb# with which the error rate is minimized is shown.
As shown, the optimum focus balance value with which the RF jitter value is minimized and that with which the ADdress In Pre-groove error rate is minimized are different. It has conventionally been considered that there are no difference in characteristics between reading ADdress In Pre-groove information and normally reading pits, and therefore the same focus balance value has been employed. However, as described above, it has been found that the characteristics may be different after information has been recorded on an optical disc.